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uscarb

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Registered: 04/18/11
Posts: 6
Reply with quote  #1 
Tech Bulletin

How much propane will I use compared to gasoline?  How long will the propane last in a 20# grill type cylinder on a generator?
Master Tech

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Registered: 04/19/11
Posts: 108
Reply with quote  #2 

This Information is taken from several pages of our website.

First from the http://www.uscarb.com/faq's.htm page of our site:

HOW MUCH PROPANE WILL AN ENGINE USE INSTEAD OF GASOLINE?

The estimate given by the manufacturer for the gasoline run time can be used for propane also because it is just that; an estimate or guess. Propane only has about 10% less Btu content than gasoline, gallon-to-gallon, so the manufactures gasoline estimate will be very close for propane. However, it is not uncommon that one may use less propane than gasoline. This can be contributed to the fact that the gasoline carburetor is jetted and nonadjustable whereas the propane system is infinitely and easily adjustable. This allows the system to be "calibrated" to the elevation it's being used and especially important for travelers using a generator at different elevations.

The following is from the http://www.uscarb.com/propane_usage.htm page:

Here we explain propane consumption, cylinder vaporization, and the size cylinder required to run a certain horsepower generator.

Propane Consumption Rate

Using some basic factors, round numbers and some good estimates, this is how run time values of propane cylinders can be worked out:

BASIC FACTORS

It requires 2 horsepower to produce 1000 watts of energy per hour

Under load, each horsepower consumes 10,000 BTU per hour

Propane contains 92,000 BTU per gallon

Propane weighs 4.2 pounds per gallon

Cylinder Capacities in Gallons and BTU's

Size

Gallon Capacity

Total BTU Capacity

20#

4.8

441,600

30#

7.1

653,200

40#

9.5

874,000

60#

14.3

1,315,600

100#

23.8

2,189,600

200#

47.2

4,342,400

420#

99.1

9,117,200

Using these factors we can arrive at run times based on average load for any generator.

For instance:

How long would a 5000 Watt Generator with a 10 HP engine at 50% load run on a 20# propane cylinder?

10hp at 50% load would be using 5 horse power to generate 2500 watts of energy.

5hp x 10,000 btu would consume 50,000 btu per hour.

Using a 20# cylinder that produces 441,600 total btu, the engine consuming 50,000 btu per hour would run for about 8.8 hours.

BTU consumption chart based on generator/engine size and load

Generator Wattage

Engine Horsepower

Full Load

75% Load

50% Load

1850

3.5

35,000

26,250

17,500

4000

8

80,000

60,000

40,000

5000

10

100,000

75,000

50,000

7500

15.5

155,000

116,250

77,500

8000

16

160,000

120,000

80,000

10,000

20

200,000

150,000

100,000

12,000

24

240,000

180,000

120,000

Many people want to know what size cylinders they need based on their engine size. Here are some real conservative estimates of the vaporization rate of various size cylinders based on the outside temperature.
 
Vaporization is discussed next...


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Master Tech

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Registered: 04/19/11
Posts: 108
Reply with quote  #3 

Continued from the http://www.uscarb.com/propane_usage.htm page of our website:

 

Vaporization Rates of Cylinders

Output in BTU's per hour - Vertical Cylinder 25% full - Minimum Cylinder Pressure 10 PSI

Cylinder Size

Outside Temperature

20

30

40

100

200

420

+60F

24,000

32,000

40,100

79,700

125,900

185,500

+50F

21,200

28,300

35,500

70,600

111,500

164,300

+40F

18,450

24,700

31,000

61,500

97,200

143,100

+30F

15,700

21,000

26,400

52,400

82,800

122,000

+20F

13,000

17,300

21,800

43,300

68,400

100,700

+10F

10,250

13,700

17,200

34,200

54,000

79,500

0

7,500

10,000

12,600

25,000

39,500

58,300

-10F

4,780

6,400

8,000

16,000

25,300

37,100

-20F

2,050

2,700

3,400

6,800

10,700

15,900

For the physical properties of each cylinder, click on the "Cylinder Size" above.

What does all this mean?

Well, if you went exactly by the chart, you would need a 420# cylinder to run a 14hp engine if it was 25% full and 40 degrees outside and keep a minimum of 10 psi in the cylinder. This is a worst case scenario. For instance, when a 20# cylinder is full it can run a 16hp engine for quite some time in 40 degree weather before there will be any freezing problem. But if you wanted to use up all the gas in a cylinder, it would have to be sized according to the chart.

Here is why. Propane is stored as a liquid under pressure and boils to produce a vapor that is drawn off at the top for the engine to use as the fuel. Because propane boils at -44° (below zero), the gas will freeze if it cannot absorb enough ambient heat to compensate for the boiling process. The bigger the cylinder is compared to the amount of load, the warmer it is outside, the warmer the cylinder is kept, all are a determining factor in the likelihood of a cylinder freezing up.

If a sweat or frost line forms around the cylinder at the level of the fuel, this is a telltale sign that the cylinder over worked and is in the process of freeze up. If the gas does freeze, it will stop producing vapor and the pressure inside the cylinder will drop to as low as zero psi which will cause the engine to stop running.

To compensate for an undersized cylinder, two cylinders can be tied together using a tee check and pigtails. If the cylinder is kept warm there should be no problem using smaller tanks on bigger engines. The method of warming a cylinder should never allow the cylinder to overheat and cause the relief valve to check off.

 


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josstark

Registered: 08/24/11
Posts: 1
Reply with quote  #4 
I have come across sources (http://en.wikipedia.org/wiki/Horsepower) which state the number of watts per horsepower is closer to 750.  Not sure if you agree with this number for the purpose of propane consumption calculations.
Master Tech

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Posts: 108
Reply with quote  #5 

That is interesting.  However, it would not seem to be in line with what is standard in the industry.  If true, it would mean that all generators have severely over-sized engines.  At 750 watts per horsepower a 7500 watt generator would only require a 10hp engine when most come standard with a 15hp engine.  Seems that 7500 watts divided by 15hp comes out to 500 watts per horsepower.  Can't say much more as to why that number is used but it may be related to something other than gas driven alternators.


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MegaOne302

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Registered: 06/25/13
Posts: 21
Reply with quote  #6 

746w = 1hp


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Furd

Registered: 10/12/12
Posts: 52
Reply with quote  #7 
746 watts equals one ELECTRICAL horsepower.  That does not take into consideration any inefficiencies.  Small generators running on single (or even two) cylinder internal combustion engines will require about 2 to 2.2 horsepower per kilowatt electrical output.
MegaOne302

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Registered: 06/25/13
Posts: 21
Reply with quote  #8 

That makes sense; my gen is rated at 3200w / 4000w max and is has a 6.5hp engine.

 

So if 2.0hp = 1kw, then 6.5hp = 3.25kw or 3250w.

 

Good to know, thank you.


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